In the prior art, various solutions are already known in which the terminals belonging to a mobile radio network are located on the basis of the physical signals and of the information available in the network, that is, without the aid of external equipments or systems, such as satellite positioning systems.
The various known solutions, aimed at allowing to locate mobile terminals on the basis of the physical signals and of the information available in the network, are distinguished from each other by the combination of two key aspects: the type of data provided to the position calculation system and the processing methodology applied to said data.
In regard to the first aspect, there are at least four types of physical quantities that can be measured by the mobile radio network and/or by the mobile terminal in order to obtain information useful for locating purposes.
In the first place, it is possible to measure the power received by the mobile terminal from a certain radio base station (currently indicated as BTS in the case of GSM and similar systems). This solution allows to obtain a circumference—centred on the base station under consideration—whereon, in an unknown point, lies the mobile terminal. Combining multiple power measurements and then intersecting the respective circumferences, the point where the mobile terminal lies can be identified. Power measurements are intrinsically inaccurate, because they are influenced by many uncontrollable factors, the most significant of which are antenna gain and fading, a phenomenon involving electromagnetic wave propagation, which cause the random and unpredictable fluctuation of the signal level.
It is also possible to measure the Timing Advance (TA), i.e. the “time of flight” of a reference signal such as a control burst between the base station and the mobile terminal (downlink) and, symmetrically, between the mobile terminal and the base station (uplink). Therefore, the value of TA indicates the distance between the mobile terminal and a base station. Like power measurements, TA also identifies a circumference whereon the mobile to be located lies. Combining multiple TA measurements (and then intersecting the corresponding circumferences) it is possible to identify the point where the mobile terminal lies. In the case of GSM and GPRS networks, TA measurements are inaccurate, both because of the ways by which they are taken, and because of the quantization error due to the finite number of bits used to store the measurement in the radio base station: in practice, the measure of TA allows to identify annuli with radial extension of about 550 m.
There are also the measurements of Observed Time Differences (OTD), obtained by measuring the difference between the distance from a mobile terminal and a base station and the distance from the same mobile terminal and an another base station. The OTD measurements describe hyperbolas that, appropriately combined, allow to locate the mobile terminal. The OTD measurements provide results that are intrinsically more precise than the two described above, because they are based on the measurement of the difference of the “times of flight” of an electromagnetic field (as evidenced by the fact that the GPS system, universally known as the most accurate positioning system currently available, is based on the same type of measurements).
Lastly, there are the measurements of Time of Arrival (TOA), entirely similar to the OTD measurements with the difference given by the fact that the measurement is taken by the network and not by the mobile terminal.
Both OTD measurements and TOA measurements have the drawback derived by the fact that, to yield accurate result, they require an exact synchronisation between the base stations: this condition requires to be achieved the presence, within the network, of additional synchronisation devices.
The four types of measurements described above are used to calculate the position of a mobile terminal both operating in an absolute way, i.e. intersecting the geometric loci described by the measurements taken, and comparing the available measurements with maps prepared a priori.
In the prior art there are different systems based both on the first method (power) and on the second method (TA), which are further differentiated by the type of measurements whereon the locating operation is based.
For instance, in U.S. Pat. No. 5,613,205 the position of a mobile terminal is estimated by intersecting the geometric loci derived from the combination of OTD and power measurements.
In WO-A-0018148 and U.S. Pat. No. 6,167,274, in order to locate a mobile terminal, the measurements of the power received by the mobile from a certain number of base stations are compared with a database which contains the power “signatures” of a certain area as a function of the geographic co-ordinates.
However, locating systems currently available in the art leave three fundamental issues unresolved.
In the first place, it is not considered that real locating scenarios are affected by measurement errors of various kinds, which have a considerable impact on locating accuracy (solutions like the one described in the document U.S. Pat. No. 5,613,205 in fact consider only some of the more relevant errors). Among the main errors to be considered are those made in geo-referencing the base station (typically in the order of a few tens of meters with peaks in the order of hundreds of meters), in measuring the times OTD and TOA due to the lack of synchronisation of the base stations (typically with geometric equivalents in the order of tens of meters), in measuring the power received by a mobile due to antenna gains and fading and, lastly, in measuring all mentioned parameters due to the systematic and intrinsic errors of the measurements themselves and to the multipath of the physical signals.
As a consequence of these errors, known locating systems yield poor accuracy. Moreover, for the methods based on the intersection of geometric loci, the various measurements can also diverge, preventing the estimation of the mobile terminal position: in fact, due to the movement of the geometric loci caused by the errors, there can be either no intersection or more than one. In the second place, for the methods that are based on the comparison between the signals received by the terminal and a database of “geographic signatures” of the signals, it is necessary to constantly update the database as the mobile radio network evolves. This updating operation is far from simple and the common risk is to compare the received signals with an obsolete database. Moreover, for practical reasons, the database is built using data calculated with mathematical models. Even in the best cases, this entails a difference relative to the values measured by the terminal in the field, and is another source of errors (see for instance U.S. Pat. No. 6,167,274).
Lastly, the methods presented in the literature and commonly known are not able to combine all types of measurements (power, TA, OTD and TOA) in a flexible way, but are limited at most to combine them in a rigid fashion, for instance, OTD measurements and power measurements (see U.S. Pat. No. 5,613,205). Consequently, when the prescribed measurements are not available, the locating system is incapable of adapting itself to the real measurement scenario, which it has to deal with, and therefore it is not able to perform its functions.